Papadopoulos had cooperated for more than a year with the probe being led by special counsel Robert Mueller in to possible collusion.

During the election he was a foreign policy adviser to Mr Trump, and relayed to the campaign that he had been told by a Maltese academic, Joseph Mifsud, that the Russians had "dirt" on Hilary Clinton in the form of thousands of emails.

Papadopoulos also suggested to the Trump campaign that he could set up a meeting with President Vladimir Putin.

US authorities were alerted in mid-2016 after Papadopoulos told an Australian diplomat, during a drinking session in a London pub, about his meetings with Professor Mifsud.

The diplomat told US investigators, but Papadopoulos then lied and said his contact with the professor happened before he joined the campaign.

US District Judge Randolph Moss, sentencing, noted that he "lied in an investigation that was important to national security."

A demonstrator outside courtCredit:
Bloomberg

The judge said he took into consideration Papadopoulos's "genuine remorse" in issuing the light sentence, which included a $9,500 fine, a year on parole and community service.

By lying to investigators, Papadopoulos had made "a calculated exercise of self-interest over the national interest," said the judge.

In cooperating with Mr Mueller's investigation Papadopoulos has said that senior Trump campaign members encouraged him during 2016 to build ties with Russia.

Out of the 35 people and entities so far charged in the probe Papadopoulos is one of five who have pleaded guilty, and the second to be sentenced.

Mr Trump has regularly lashed out against the sprawling investigation, which he dubs a "witch hunt" driven by his Democrat enemies.

George Papadopoulos at a meeting with Donald Trump in March 2016Credit:
AFP

Papdopoulos's lawyer Tom Breen said "the President of the United States hindered this investigation more than George Papadopoulos ever did".

Papadopoulos, from Chicago, was a petroleum analyst based in London when he joined the Trump campaign in March 2016 as one of a handful of members of the Republican candidate's national security and foreign policy advisory board.

Within weeks he had made contact with Prof Mifsud, who introduced him to others including a woman who claimed to be Mr Putin's niece.

At the end of March 2016 Papadopoulos told Mr Trump, then-senator and now Attorney General Jeff Sessions, and members of the national security team at their first meeting in Washington, that he had connections in London who could set up a Trump-Putin meeting ahead of the November election.

"While some in the room rebuffed the offer Mr Trump nodded with approval and deferred to Mr Sessions, who appeared to like the idea and stated that the campaign should look into it," Papadopoulos later claimed in a statement to a court.